1. Field of the Invention
This invention relates to an optical fiber for transmitting laser energy and, in particular, to an optical fiber that is suited to transmit high-output laser light with an infrared wavelength band which is useful for medical applications. This invention also relates to a laser energy transmission method and a laser energy transmission device using the optical fiber.
2. Description of the Related Art
In recent years, according as semiconductor lasers or solid-state lasers are provided with a higher output or shorter pulse, various uses have been developed using laser energy with high peak power. Especially, the use for laser processing or a laser surgery attracts attention.
Conventionally, a silica optical fiber 30 with a large diameter as shown in FIG. 1 is generally used to transmit a high output laser light. It is structured such that, like optical fibers as used for communications, the laser light is transmitted concentrating its optical power in a core region 31 while satisfying the total internal reflection condition at the boundary of the core region 31 with a relatively high refractive index and a clad region 32 with a low refractive index.
In general, such a silica solid-type optical fiber can transmit a laser light of about 2 μm or less wavelength at a low loss. The silica solid-type optical fiber for high power transmission is different from the general optical fibers for communications in that single-mode transmission is not needed and the power density of transmitted laser light is reduced using a large core with a diameter of 100 to 600 μm. Since the silica solid-type optical fiber can transmit a laser light of about 2 μm or less wavelength at a low loss for a long distance, it is mainly used to transmit a neodymium (Nd)—YAG laser light (with a wavelength of 1.06 μm) which has high oscillation efficiency and high output, and it is in practical use for a laser processing or as a laser surgery for hemostasis. On the other hand, a holmium (Ho)-YAG laser light (with a wavelength of 2.1 μm) can be barely transmitted using the silica solid-type optical fiber for a distance of less than a few meters. The Ho-YAG laser light is taken into account mainly for medical use since it has a relatively high absorption to water.
When the wavelength of laser light is more than 2 μm, the silica solid-type optical fiber cannot be used for a long distance transmission. An erbium chrome (ErCr)-YSGG (=Yttrium Scandium Gallium Garnet: Y3Sc2Ga3O12) laser light (with a wavelength of 2.78 μm) and an erbium (Er)-YAG laser light (with a wavelength of 2.94 μm) are effective for cutting of a hard tissue such as a bone or teeth since they are well absorbed especially by water, e.g., very efficiently absorbed by a hydration shell between hydroxyapatite crystals. However, the laser light with a wavelength near 3 μm cannot be transmitted through the silica solid-type optical fiber. Therefore, a hollow fiber with a hollow region as a core is used therefor.
As shown in FIG. 2, the hollow fiber 40 comprises a hollow-core region 41, a dielectric layer 42 defining the hollow-core region 41 inside thereof, and a metal layer 43 formed surrounding the dielectric layer 42. The thickness of the dielectric layer 42 is optimized according to a wavelength of transmitted laser light to enhance the reflectivity of the inside wall to allow a low-loss transmission. It is mainly applied to laser therapy equipment for dentistry.
The related art of the invention is, for example, JP-A-2000-35521 and JP-A-2002-541507.
In general, the silica optical fiber is sufficiently transparent to light with a wavelength of less than 2 μm and is operable to transmit it at a low loss for a long distance. However, even in the case of a laser light with a wavelength of less than 2 μm such as a titanium (Ti)-sapphire laser light of a wavelength near 0.8 μm and a neodymium (Nd)-YAG laser light of 1.06 μm wavelength, the core region may be broken since its peak power can be extremely increased spatially and temporally when the laser light has a high output and short pulse. This is because, due to an increase in laser energy by nonlinear optical effect, the refractive index increases to allow the self-focusing of light.
Therefore, as shown in FIG. 1, the conventional laser energy transmission optical fiber needs to have the enlarged core diameter such that it can be used within such a power density as not to exceed a breakage threshold of the optical fiber. Thus, there is a problem that the conventional laser energy transmission, optical fiber is subjected to a limitation in transmission capacity of laser energy and in mechanical flexibility.
In recent years, a concept has been suggested that, in the case of a laser light with a very high power density as described above, a hollow core is advantageous to transmit the laser light even when its wavelength is 2 μm or less. This is because the breakage threshold of end face can be significantly enhanced without using the enlarged diameter. A transmission line suited for this concept is a photonic bandgap fiber that has a hollow central portion with plural holes formed outside thereof. The photonic bandgap fiber can have a bandgap to a specific wavelength depending on the hole diameter and the interval between the holes to offer a low-loss transmission while confining light in its hollow-core region.
However, since the bandgap is designed to be formed to a specific wavelength band to allow the low-loss transmission of a specific wavelength laser light such as a Nd-YAG laser light, laser light in another wavelength band cannot be always transmitted at a low loss. In practice, a visible laser light such as green or red light needs to be superposed as a guide light since the Nd-YAG laser light is invisible. Thus, although the conventional photonic bandgap fiber is excellent in transmission of only the specific laser light, it is not suited to transmit a visible light as the guide light together with the laser light.
On the other hand, in the case of an ErCr-YSGG laser light or Er-YAG laser light, the hollow fiber with the dielectric formed inside thereof (herein also called dielectric-formed hollow fiber) as shown in FIG. 2 is used since it is not possible to use the solid-type silica optical fiber. Also in this case, the visible light needs to be transmitted being superposed as the guide light. However, since the thickness of the dielectric layer 42 is set to allow the low-loss transmission of the specific infrared laser light, a large loss occurs in the visible laser light. When it is transmitted a few meters, the output of guide light may be recognized but a laser light source with high power needs to be used therefor.
As described earlier, laser light with a wavelength near 3 μm is well absorbed by water and, therefore, it is an important light source for medical use. Especially, a laser therapy intended to minimally invasive treatment attracts attention recently. In this use, the dielectric-formed hollow fiber with a diameter of about 500 to 1000 μm is generally used since it is difficult to make it a very narrow fiber with a diameter of about 100 μm.
Since the dielectric-formed hollow fiber has the hollow core, when its tip accesses or contact the affected part or it is inserted into the body, deterioration in optical or mechanical characteristics may occur such as an invasion of foreign material into the hollow-core region, an increase in loss due to the contamination of the tip portion and a breakage failure. Further, since it is difficult to clean or sterilize, it is not possible to reuse it.
A suggested solution for sealing the hollow part is attachment of a silica end chip to prevent the invasion of foreign material and to control the spread angle or direction of light outputted (K. Iwai, Y. Shi, M. Endo, K. Ito, Y. Matsuura, M. Miyagi and H. Jelinkove, App. Opt., vol. 43, pp. 2568-2571, 2004). It is structured such that the tip of the dielectric-formed metallic hollow fiber 40 is covered with the end chip, like a cap, to seal the hollow-core region 41. Since the dielectric-formed metallic hollow fiber 40 is composed of metal and dielectric film, the chip cannot be integrated by fusion-bonding. Therefore, the tip portion needs to be enlarged and the chip may fall off when the fiber is inserted into the affected part.